Shrinking patterns and phase coexistence of polymer gels

We have investigated the shrinking phase transition of slightly ionized poly(N-isopropylacrylamide) (NIPA) gels. The macroscopic conformation change was observed on the heating process in two different methods; a continuous heating process and an isothermal process. It was found that the macroscopic behavior can be characterized by several conformation changes, the phase coexistence (or 'linked-dumplings'), the grain pattern, the bubble pattern, and the opaque phase. Those have correlations with the phase transition velocity. The heating conditions to determine the characteristic conformations and the stability of the phase coexistence were qualitatively discussed in terms of the classical phase separation model of nucleation and spinodal decomposition. (C) 2000 Elsevier Science Ltd. All rights reserved.